This is the first competitive renewal of grant for which the long term objective is to characterize the role of LAR in regulating cell-matrix interactions. LAR is a transmembrane, receptor-like protein tyrosine phosphatase (PTP), first isolated by the investigator, that displays features of an immunoglobulin superfamily cell adhesion molecule in its extracellular segment. In the course of the initial funding period, the investigator demonstrated several properties of LAR that were consistent with such a role. For example he demonstrated that; LAR is localized to regions of association between epithelial cells and basement membrane in diverse tissues; LAR and the intracellular, coiled-coil, LAR-interacting protein LIP.1 colocalize to focal adhesions; LAR binds a multidomain protein termed Trio which contains a rac-specific guanine nucleotide exchange factor (GEF) domain and a rho-specific GEF domain; and LAR-deficient fibroblasts derived from LAR-knockout mice have altered cell morphology. The investigator proposes to build on this foundation to test further the hypothesis that LAR regulates cell-matrix interactions by addressing the following specific aims. 1) To determine the role of the LAR binding protein LIP.1 in LAR-mediated signaling; 2) To determine the role of the LAR binding protein Trio in LAR-mediated signaling; 3) To identify the LAR substrate; and 4) To identify the extracellular ligand(s) of LAR. The investigator proposes to achieve these aims by: (i) characterizing cell lines overexpressing LIP.1 and characterizing LIP.1 binding proteins that may link LAR to other proteins including LAR substrates; ii) characterizing the LAR-Trio association in vivo, determining the effect of LAR expression on Trio transforming activity, and characterizing LAR-deficient cells and Trio-transformed cells; iii) identifying a protein that is hyper-tyrosine phosphorylated in LAR-deficient cells; and iv) purifying and characterizing LAR extracellular region-binding proteins, and determining the functional effect of LAR ligand binding.